This invention relates to an amplifier system, especially an adaptive amplifier system for a signal receiver, for example a portable telephone handset or satellite receiver.
A radio signal receiver typically includes a chain of filter stages and amplifier stages ("gain stages") through which the received radio frequency signal is passed in series. The filter stages and amplifier stages alternate throughout the chain. The filters filter out unwanted (out of band) frequencies and the amplifiers amplify the remaining signals. The resulting signal passes to a mixer where it is downconverted and it is subsequently demodulated.
Signal receivers that are especially sensitive (those having a high degree of amplification through the chain) may suffer particular problems. To cope with the extreme sensitivity of the signal the receiver must have a very low noise characteristic (known as noise figure); otherwise the noise will tend to be amplified so that it swamps the signal. However, to achieve the required sensitivity relatively many gain stages may be required: possibly three or more gain stages in the receiver's UHF section alone, whereas a typical conventional mobile telephone has only one or two gain stages. And if a three gain-stage UHF section, for instance, is built using the same principles as a standard receiver section its IIP3 ("third order input intercept point") performance will be relatively poor (perhaps 20 dB worse than that of a comparable signal receiver with only one or two gain stages). Therefore, such a signal receiver will be especially vulnerable to interference from unwanted signals--especially signals just outside the band of desired frequencies. The problems will be particularly acute if the signal receiver's filters are low insertion loss filters with relatively little attenuation of signals just outside the desired band.